1. Field of the Invention
The present invention relates to a superconducting cable of the so-called high temperature type, and a manufacturing process for the same.
2. Description of the Related Art
The term superconducting material, as employed throughout the description and the appended claims, refers to any material, such as for instance ceramic materials based on mixed oxides of copper, barium and yttrium or of bismuth, lead, strontium, calcium, copper, thallium and mercury, comprising a superconducting stage having an almost null resistivity at temperatures below a so-called critical temperature Tc.
In the field of superconductors and, accordingly, in the present description, the term high temperature refers to any temperature near to or higher than the temperature of liquid nitrogen (about 77xc2x0 K), compared to the temperature of liquid helium (about 4xc2x0 K), usually indicated as low temperature.
High temperature superconducting cables are known for instance from DE-A-3811050 and EP-A-0747975.
Superconducting materials are known that have a critical temperature higher than 77xc2x0 K, i.e. that show superconductivity characteristics at least up to such temperature. These materials are usually referred to as high temperature superconductors. Such materials are obviously of a greater technical interest with respect to low temperature superconductors, as their working may be ensured by liquid nitrogen refrigeration at 77xc2x0 K instead of liquid helium at 4xc2x0 K, with much lower implementation difficulties and energy costs.
As known, in the field of electric energy transportation, one of the problems of most difficult solution is that of rendering more and more advantageous the use of the so-called superconducting materials, from both the technological and the economic points of view.
In fact, even though these low temperature materials have been known for a long time, their diffusion was limited till now to some well defined practical applications, such as for instance the fabrication of magnets for NMR apparatuses or high field magnets for which cost is not a discriminating factor.
Actually, cost savings due to less power being dissipated by superconductors is still more than counterweighted by costs due to liquid helium refrigeration, necessary to keep the latter below its critical temperature.
In order to solve the aforesaid problem, research is partly oriented towards experimenting new high temperature superconducting materials, partly tries to constantly improve both the characteristics of the existing materials and the performances of conductors incorporating already available materials.
With regard to geometric characteristics, it has been found that an advantageous geometry is provided by thin tapes having, generally, a thickness between 0.05 mm and 1 mm.
In fact, in such case the conductor comprising the very brittle superconducting ceramic material achieves on one hand an improved resistance to various bending stresses to which it is submitted during each manufacturing, shipping and installing operations of the cable containing it, and on the other hand it provides better performances with regard to critical current density, because of the more advantageous orientation and compacting degree of the superconducting material.
For various reasons and in particular to improve mechanical resistance, the above conductors generally comprise a plurality of tapes, formed each by a core of superconducting material enclosed in a metal envelopexe2x80x94generally of silver or silver alloysxe2x80x94coupled together to obtain a multi-filament composite structure.
According to a widely used method, known to those skilled in the art as xe2x80x9cpowder-in-tubexe2x80x9d, this multi-filament structure of the conductor is obtained starting from small metal tubes filled with a suitable powder precursor, said tubes being in their turn enclosed in another external metal tube or a billet, so as to obtain a compact bundle of tubes which are submitted first to several subsequent permanent deformation, extrusion and/or drawing treatments, then to rolling mill and/or pressing treatments, until the desired tape-shaped structure is obtained. See for instance EP-A-0627773.
Between a rolling mill treatment and the subsequent one, the tape being worked is submitted to one or more heat treatments to cause formation of the superconducting ceramic material starting from its precursor and, above all, its syntherisation, i.e. the mutual xe2x80x9cweldingxe2x80x9d of the granules of the powdered superconductor.
The tapes of high temperature superconductors are rather brittle, both at the working temperature of 77xc2x0 K and at room temperature, and are unsuited to stand mechanical stresses, especially tensile stresses. In fact, apart from an actual mechanical breaking, exceeding a given tensile deformation threshold irreversibly jeopardises the superconduction characteristics of the material. Therefore, using these materials in cables is particularly complex and delicate.
In fact, the manufacturing and installation of cables comprising such materials involves several stages which bring about unavoidably mechanical stresses.
A first critical stage is winding of several tapes on a flexible tubular support according to a spiral arrangement, until the desired section of superconducting material is obtained. Both winding and pull cause tensile, bending and torsion deformations in tapes. The resulting stress applied to the superconducting material is mainly a tensile stress. Besides, the so formed conductor (support plus superconducting material) is surrounded by heat and electric insulation means, and is submitted, during these operations, to tractions and bendings that introduce more stresses in superconducting tapes.
A second critical stage concerns cable installation. The cable is installed at room temperature, which causes additional tensile and bending stresses. Mechanical connections (i.e., locking of cable heads), electric connections, and hydraulic connections (i.e., for liquid nitrogen) are carried out at room temperature. After completing installation, the cable is brought to its working temperature by feeding liquid nitrogen. During such cooling, each cable component is subject to mechanical stresses of thermal origin, differing according to the thermal expansion coefficient of the constituting material and of the characteristics of the other elements.
In particular, the differences of expansion coefficients between the support and the superconducting tape may cause stresses in the latter and therefore in the superconducting material. In fact, if the superconducting material cannot shrink freely being tied to a less shrinkable support, tensile strains generate in the superconducting material. Such tensile strains add to those already present, due to winding.
To reduce tensile strains, the use of supports has been suggested that are made of a material having an expansion coefficient higher than that of the superconducting material (usually equal to 10xc3x9710xe2x88x926/K-20xc3x9710xe2x88x926/K), i.e., on the order of at least 75xc3x9710xe2x88x926/K. Such material would not be a metal, as no known metal has such values, but only a polymeric material such as, for instance, Teflon(copyright), polyethylene, and derivatives thereof.
However, it has been found that the aforesaid solution, whose aim is to reduce thermomechanical stresses on tapes through a suitable reduction in the support diameter, shows some important drawbacks.
In particular, the unavoidably high values of heat contraction of the conductor (support plus superconducting material) cause the formation of a wide radial hollow space between the conductor itself and the surrounding insulating means (thermal and/or electric insulation). This hollow space may cause electric inconveniences, with deformation or breaking of the insulation, and/or mechanical inconveniences, namely lack of cohesion, misalignment and slipping of the conductor.
Besides, poor mechanical characteristics of said polymer material do not allow to protect the superconducting material adequately during cable manufacturing and installation phases: because of the high deformability of these materials any strain applied to conductors causes indeed a remarkable deformation also in the superconducting material.
Therefore, the invention relates, in a first aspect, to a high temperature superconducting cable, comprising a tubular support, a plurality of superconducting tapes including a superconducting material enveloped in a metal covering (for instance, silver or a silver-based alloy with magnesium and/or aluminium and/or nickel), said tapes being spirally wound on the support, so as to form an electroinsulated, thermally-insulated and refrigerated superconducting layer, characterised in that the superconducting tapes have a maximum tensile deformation greater than 3‰.
The above value is to be intended as referred to the manufacturing and installation process described above, i.e.: winding and installation at room temperature, then cooling to working temperature of about 77xc2x0 K. The same applies also to the deformation values that will be given in the following.
Preferably, the superconductive tapes comprise at least a metal strip (or band or laminate) connected to the metal covering.
In this way, the capability of bearing tensile stresses increases. It has been observed that tensile deformation safely bearable by superconducting materials may bexe2x80x94at bestxe2x80x94about 3‰. This figure takes into account the fact that the superconducting materials already bear a compression deformation of about 1‰-1.5‰ because of the different thermal contraction of the superconducting material relative to the metal covering during the tape fabrication stage.
Thanks to the metal strip of the invention, not only a lower deformation under the same applied strains has been observed, but especially an improved tensile deformation resistance; elongation values equal to about 5.5‰ have been actually reached without any damage. This effect is thought to be due to a more uniform distribution of strains in the superconducting material, that allows to better exploit the mechanical characteristics of said superconducting material.
According to each individual case, only one strip coupled to the metal covering, or two strips, located at the opposite sides of the tape, can be provided.
Preferably, the metal strip is coupled to the metal covering by welding, brazing or gluing.
Preferably, the strip is made of non magnetic stainless steel having a low electric conductivity, or also of bronze or aluminium.
Preferably, the tubular support of the cable is made of metal. The greater capability of bearing tensile stresses allows indeed to use a support made of metal instead of polymeric material, as will be better explained in the following.
Various types of metals may be used for the support; in particular, for applications with very high currents, non-magnetic steel is used, preferably stainless steel. Alternatively, copper or aluminium may also be used.
The structure of the tubular support may be continuous, either smooth or corrugated. Alternatively, the tubular support may have a structure formed by a spirally wound metal tape, or may have a so-called tile-structure, i.e. with spirally connected adjacent sectors.
In a second aspect, the invention relates to a process for manufacturing high temperature superconducting cables, comprising the steps of:
providing a tubular support,
enclosing a superconductive material in a metal covering, so as to form superconductive tapes,
spirally winding a plurality of superconducting tapes onto the support so as to form at least a superconducting layer,
electroinsulating the superconductive layer,
thermally insulating the superconductive layer,
providing the possibility of refrigerating the superconductive layer below a predetermined working temperature, when cables are in use, characterised by
controlling the maximum tensile deformation of the superconducting tapes to have it greater than 3‰.
This process allows to manufacture cables according to the first aspect of the invention.